I'll just briefly go over important stuff and choices

Priority: Maxing out rising storm, cc big damage, helps with farming, bread and butter of musa, the final upgrade is extra damage but on a long CD (maxing dash/sp restore is up here too)

2nd priority: Gale/dragons bite skill lines, you can animation cancel these together with your dash to get an insta-tornado for big damages, once you get the combo down its deadly. (Also don't forget blooding + blooding phantom, can be cancelled for good cc/dmg, a MUST have)

3rd: blind thrust, twisting slash skill lines (twisting better imo) are a good investment and will pay out farming and with your rising storm combo chains, tempest is quick and big dmg.

Final priorities: Carver is optional and you could even argue its better than putting points into blind slash, I liked using it and I upgraded it all the way first in korea, but it doesn't fit everyone. The bow skills are all super low damage and if you are going to invest, put points into the charged shot for the cc, other than that the last tree is an okay dodge but rather hard to aim and has a long cd. Blinding slash also a good upgrade if you are quick enough to get the air attack (which are 2.5x dmg I believe) from nemesis; personally I find it a harder combo to pull off and I never used it much, that's why the blind thrust tree is low on my list in general.

Skills to stay away from:

Tiger blade, I know this is a super fanboy skill and I'm sure some people have arguments defending it. Not me, in short it cuts your skill tree off and trades it for only being able to dash and auto with lifesteal (yes you have a few other skills, but they are not good ones) and imo its just a waste of SP you can put to better use elsewhere.

Backstep slash: personal dislike for the skill, it has a slow animation and pushes you backwards. While it gives you a slow it is just a very awkward skill that never finds its way in a real combat scenario. If your enemy is open you have much higher damaging skills, if you are trying to get away this skill is subpar because it only moves you backwards but gives you neither a block or an I-frame, quick both your block...AND dash give you, so its a waste in my book, the animation doesn't justify the cc slow for me, maybe at the end of a combo but even then you are taking a risk.

Upgrading block: Dislike very much, the original un-upgraded version seems to have more pep to it without the upgrades, and the upgrades lock you into an animation that is very easily read/dodged and leaves you vulnerable and unable to dash for 1s or so (big deal on a glass cannon) the damage modifier and cc seem attractive on paper but in practice this skill got me killed far more than it actually worked out for me, so I got rid of it :)

Divider: if you are only farming and hoping for no pvp encounters, go ahead and upgrade divider for the health return, it's totally useless in PVP (except using it for animation cancelling your dash, which 99% of the time, I would not advise)

Upgrading kicks: Screws up your dash animation cancel, hopefully they do not dumb down the way this works from Korea (hush crybabies)

Auto attack (slice) : as with most classes, there is pretty much no situation where you will be standing still and pressing lmb in real combat, and even if you had the opportunity carver: typhoon is still more damage, so...why? I'd say this is the last thing you should upgrade (talking like 900 SP here) just to get the bleed so you can try to weave in that one auto for the -15dmg per 3s.

Arrow grapple: go upgrade it in game, seriously. only actually using this skill can describe how awkward it is.